1. Field of the Invention
The present invention relates to apparatus, methods and systems for increasing performance and stability of fixed cylindrical elements subjected to flowing fluid environments. In another aspect, the present invention relates to apparatus, methods and systems for reducing drag and vortex-induced vibrations (VIV) on cylindrical elements in marine environments.
2. Description of the Related Art
Production of oil and gas from offshore fields has created many unique engineering challenges. These challenges include dealing with the effects of ocean currents on fixed cylindrical marine elements, and dealing with the effects of wind currents on fixed cylindrical elements exposed to the atmosphere.
With respect to fixed cylindrical marine elements, these elements are used in a variety of applications, and include subsea pipelines; drilling, production, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; space-frame members for platforms; cables; umbilicals; mooring elements for deepwater platforms; and the hull and/or column structure for tension leg platforms (TLPs) and for spar type structures. Ocean currents produce drag on the cylindrical marine elements and cause vortexes to shed from the sides of the elements, thereby inducing drag forces and vibrations that can lead to the failure of the elements. The large drag forces result in increased mooring or station keeping costs, and impose constraints on what kinds of systems are workable in a given environment (due to stress limitations, top angle limitation while drilling, etc.). Large vibrations (primarily vortex-induced vibrations) cause substantial increases in mean and dynamic drag forces, and also cause dynamic motions that result in premature fatigue failures of structural members. In addition, the presence of ocean currents can cause interference between adjacent structures.
Numerous attempts have been made in the art to overcome the above set of issues. One such attempt involves the use of helical strakes. Unfortunately, helical strakes are not effective at reducing drag, and are therefore rarely used in situations in which drag reduction is important (e.g. drilling risers). Another attempt to improve performance and stability of cylindrical marine elements is the use of fairings. However, there are many instances in which the use of fairings is impractical and/or uneconomical. For example, fairings are typically quite large and quite expensive, can be very difficult to handle, and generate large installation costs.
In spite of advancements in the art, there is a need for apparatus, methods and systems for maximizing the performance and stability of cylindrical marine elements. There is another need for apparatus useful for reducing ocean current-induced drag force and vibrations on cylindrical marine elements. There is even another need for methods of installing such apparatus onto cylindrical elements. There is still another need for systems for reducing ocean current-induced drag force and vibrations on cylindrical marine elements.
These and other needs will become apparent to those of skill in the art upon review of this specification, including its drawings, claims and appendix.
It is an object of the present invention to provide apparatus useful for reducing ocean current-induced drag force and vibrations on cylindrical marine elements.
It is another object of the present invention to provide methods of installing such apparatus onto cylindrical elements.
It is even another object of the present invention to provide systems for reducing ocean current-induced drag force and vibrations on cylindrical marine elements.
One embodiment of the present invention is directed to an apparatus for reducing drag and vortex-induced vibrations on cylindrical marine elements. Generally the apparatus have a substantially cylindrical shape and comprise an interior surface and an exterior surface, wherein the exterior surface has a roughness parameter K/D of a value of about 1.0xc3x9710xe2x88x924 or less, wherein K is an averaged value of the distance between the peaks and troughs on the exterior surface, and D is the diameter of the apparatus.
Another embodiment of the present invention is directed to methods for controlling drag force and vortex-induced vibration on a substantially cylindrical element by providing an ultra-smooth surface about the cylindrical element. In general, the methods comprise the steps of: a) positioning a sleeve around a cylindrical element, wherein the sleeve covers from about 180 degrees to about 360 degrees of the circumference of the element; and b) securing the sleeve in position around the circumference of the element. In keeping with the sleeves of the invention, the sleeves used in the present methods have a K/D value of about 1.0xc3x9710xe2x88x924 or less.
Even another embodiment of the present invention is directed to systems for reducing the effects of drag and vortex-induced vibration on fixed cylindrical structures utilized in offshore operations. Generally the systems of the invention comprises a substantially cylindrical marine element and a substantially cylindrical sleeve, wherein the sleeve is secured in place around the circumference of the element. Generally the sleeve covers from about 180 degrees to about 360 degrees of the circumference of the marine element, preferably about 200 degrees to about 360 degrees, more preferably about 220 degrees to about 360 degrees, and most preferably about 240 degrees to about 360 degrees.
These and other embodiments of the present invention will become apparent to those of skill in the art upon review of this specification, including its drawings, appendix, and claims.